1. Technical Field
The invention relates to a piston pin bushing. In addition, the invention relates to a method of producing such a piston pin bushing.
2. Related Art
Piston pin bushings are bushings which are provided in the small end eye in connecting rods for internal combustion engines. In the event of heavy engine load, a problem has arisen in the form of a tendency on the part of the bushing, in particular in the middle area (relative to the radial axis of the bushing), to suffer seizure during running in of the engine.
Piston bushings may be solid and made from one material or they may comprise a backing layer and an overlay. Where an overlay is mentioned below, it is also used to mean the part of the material of a solid piston pin bushing located at the inner surface.
In the past, more attention was paid to the problem of the introduction of forces and distribution of stresses in the area of the piston/connecting rod connection.
The attempt is made in DE 30 36 062 C2 to avoid stress peaks in the upper area of the piston pin bosses by rounding the edge of the piston pin bore or widening the piston pin bore.
According to DE 41 33 586 A1, local overloading of the piston pin bushing is prevented by making resilient the end areas of the connecting rod bore, i.e. the small end eye, adjoining the end faces. To this end, either relief grooves are incorporated or the wall thickness is minimized.
In DE 198 28 847 A1, the piston pin bushing is provided with an adequate service life at relatively high operating forces in that material reductions, in particular perforations, are provided in the transition region between the apex lines of the bushing. In this way, the radial pressure is shifted to the area of the bushing ends.
According to DE 100 29 950 A1, the pressure peaks at the outer edge of the connecting rod eye, which are caused by bending of the pin under load, are absorbed in that the connecting rod eye takes the form of a shaped bore which differs from cylindrical in shape in such a way that it follows the deformation of the piston pin. The use of a bushing is thereby intended to become superfluous. Relief grooves, which are complex to produce and are therefore expensive and, moreover, lead to loss of oil, are also superfluous.
However, there are disadvantages in dispensing with a bushing: on the one hand, it is only possible to achieve optimum friction pairing between connecting rod and piston pin by using a bushing. On the other hand, the bushings also contribute to achieving alignment with the big end eye as a result of post-machining of the bushings after installation.
EP 0 716 240 B1 relates to increasing the seizure resistance of a plain bearing. This is achieved therein by a defined surface structure which extends from the bearing alloy layer through the intermediate layer and into the overlay layer. This peak and valley structure with specified conditions relating to peak height, intermediate layer thickness and overlay layer thickness has the effect that, even after a degree of abrasion, the sliding surface still includes parts of the overlay and intermediate layers, which are thus available as lubricant.
However, plain bearings differ fundamentally from bushings with regard to type of stress and lubrication conditions. Plain bearings are pressure oil lubricated and a hydrodynamic lubricating film generally forms due to the large relative speeds between shaft and bearing. In contrast, in the connecting rod small end eye generally only small, oscillating relative movements arise between piston pin and piston bin bushing, i.e. mixed friction conditions arise far more frequently, resulting in solid-solid contact. The findings disclosed in EP 0 716 240 B1 cannot therefore be directly transferred to bushings, in particular piston pin bushings.